1. Field of the Invention
The present invention relates to a semiconductor laser element used for optical communication, optical information processing, optical disk devices, optical interconnection, etc., and also relates to a method for the manufacturing the same.
2. Description of the Related Art
Semiconductor lasers have so far been used in large amounts under various environments for optical communication, optical information processing, optical disk devices, optical interconnection, etc. Due to this, it is strongly demanded to manufacture, in large amounts and at low costs, semiconductor lasers having an excellent environmental resistance and, particularly, an excellent high-temperature high-output characteristics. In order to realize this type of semiconductor laser, it is important to reduce the unnecessary leakage current which flows through the portions other than the active layer, due to which researches and development of BH (buried heterostructure) lasers having various current confinement structures are being prosecuted.
FIG. 1 is a sectional view showing a laser of a DC-PBH (Double Channel Planar Buried Heterostructure) structure as a BH laser with excellent high-temperature high-output characteristics. In FIG. 1, the reference symbol 1a denotes an n-InP substrate, numeral 3 denotes an active layer, numeral 4 denotes a recombination layer, symbol 5a denotes a p-InP blocking layer, symbol 5b denotes an n-InP blocking layer, numeral 6 denotes a p-InP buried layer, symbol 7a denotes a p-InGaAs contact layer, numeral 8 denotes an insulating film, and numeral 9 denotes an electrode. This structure is shaped in such a manner that the InGaAsP recombination layer 4 with a band gap narrower than that of InP is inserted into the current blocking layers 5a and 5b which comprise a pnpn thyristor structure of InP. The carriers which function as the gate current of the pnpn thyristor are made to emit light and recombined in this narrow band gap layer, whereby the current gain of the npn or pnp transistor constituting the thyristor is reduced, whereby the turn-on operation of the thyristor is suppressed to enhance the current confinement characteristic. The DC-PBH structure has so far been made in such a manner that, after the active layer 3 is grown flat on the n-InP substrate 1a, a mesa stripe is formed by etching, and, by the use of the LPE (Liquid Phase Epitaxy) method, a buried layer containing a current blocking layer is grown, but this manufacturing method using the measure of etching the semiconductor layer and growing the buried layer by the use of the LPE method is inferior in respect of controllability, uniformity and reproducibility.
On the other hand, semiconductor lasers each constituted in such a manner that semiconductor lasers each having the BH structure fabricated by the use of the MOVPE (Metal Organic Vapor Phase Epitaxy) method which is excellent in respect of controllability, uniformity and reproducibility are being ardently studied and developed, but, in this case, also, it is proposed to improve the high-temperature high-output characteristic by inserting a recombination layer with a narrow band gap into the pnpn blocking layer. FIG. 2 is a sectional view showing the structure of the RIB-PBH (Recombination Layer Inserted Planar Buried Heterostructure) laser using a p-type substrate 1b which laser is disclosed in Japanese Patent Application Laid-Open No. 6-338654. Further, in Japanese Patent Application Laid-Open No. 8-236858, there is disclosed the fact that, by optimizing the band gap composition and position of the recombination layer 4 inserted into the current blocking layers 5a and 5b, the high-temperature high-output characteristics can be improved.
Further, in FIG. 2, the same component portions as those shown in FIG. 1 are referenced by the same reference symbols and numerals, whereby the repetition of the detailed description thereof is omitted. Referring to FIG. 2, the reference symbol 1b denotes a p-InP substrate, symbol 7b denotes an n-InGaAs contact layer, symbol 11b denotes a p-InP cladding layer, numeral 16 denotes a first n-InP buried layer, and numeral 17 denotes a second n-InP buried layer.
However, even if buried layer growth is performed by the use of the above-mentioned MOVPE method, there remains the problem that the dispersion in element characteristics resulting from the controllability, uniformity and reproducibility of the semiconductor layer etching cannot be avoided.
In contrast, as disclosed in IEEE, Photonics Technology Letters, March 1997, Vol. 9, No. 3, pp. 291 to 293, there is proposed a method according to which, by utilizing selective growth, the mesa stripe containing the active layer is directly formed, whereby the etching of the semiconductor layer is avoided; and thus, a BH laser with an excellent high-temperature high-output characteristic can be fabricated with excellent uniformity and reproducibility.
FIGS. 3A and 3B show the sectional structure of the DC-PBH laser fabricated on an n-type substrate 1a by the use of the above-mentioned conventional method and the pattern of a selective growth mask 2 used for the growth of an active layer 3. By setting the width Wm of the selective growth mask 2 to about 3 to 10 xcexcm, the structure constituted in such a manner that a narrow band gap layer is inserted at a position apart by the mask width Wm from the active layer 3 is realized.
However, this conventional method has the drawback that it is impossible to control independently the band gap and position of the recombination layer 4 for the optimization thereof.
Further, considerable research has been focused recently on fabrication methods for semiconductor lasers, wherein the growth speed in selective etching and the dependence on the mask width of the structure are used to integrate spot-sized conversion waveguides into the semiconductor lasers.
FIG. 4A shows the spot size conversion waveguide integrated laser disclosed in Japanese Patent Application Laid-Open No. 7-283490. Such a spot size conversion waveguide integrated laser is realized by setting the mask width of a laser region 13 to several tens of xcexcm so that the difference of this mask width from the mask width at the end of a spot size conversion waveguide region 14 may become large. Due to this, as shown in FIG. 4A, the laser is constituted in such a manner that, as shown in FIG. 4A, the narrow band gap layer effective for improvement of the current confinement characteristic does not exist in the current blocking layers 5a and 5b in the vicinity of the active layer 3. As a result, a comparison between the characteristic of the laser element with the spot size conversion waveguide portion 14 removed and that of an element in which the recombination layer 4 exists in the vicinity of the active layer 3 reveals that this laser element is inferior in respect of its high-temperature high-output characteristic.
It is an object of the present invention to provide a semiconductor laser constituted in such a manner that the active layer comprised of a semiconductor layer is directly formed by selective growth and also to provide a method for the manufacturing the same, wherein a recombination layer which has an arbitrary band gap at an arbitrary position in the vicinity of the active layer can be batch-formed together with the active layer; and thus, semiconductor laser elements with excellent high-temperature high output characteristic and a method according to which the above-mentioned semiconductor laser elements can be manufactured in large amounts with excellent uniformity and reproducibility and at low costs can be provided.
The semiconductor laser according to the present invention comprises an active layer and at least one recombination layer; and the material constituting the current blocking layer recombination layer is in the shape of a stripe with a band gap narrower than that of the material constituting the current blocking layer. The active layer is formed of a semiconductor layer and is directly formed by selective growth, and the recombination layer is formed in the current blocking layer by selective growth at the same time as the active layer.
The stripe width of the recombination layer may be 20 xcexcm or less. Also, the interval between the above-mentioned active layer and the recombination stripe nearest to the active layer may be 1 xcexcm to 10 xcexcm.
Further, the present invention provides a semiconductor laser manufacturing method wherein an active layer comprising a semiconductor layer is directly formed by selective growth. A selective growth mask pattern is used, which has at least one or more pairs of openings, besides the opening for growing the active layer. These additional openings are positioned on either side of the opening for growing the active layer, to selectively grow the recombination layer.
Further, the openings other than the opening for growing the active layer are each in the shape of a stripe with a width of 20 xcexcm or less.
Further, the mask width between the opening for growing the active layer and the opening which is other than the active layer growth opening portion and is located nearest to the opening for growing the active layer is at least 1 xcexcm to 10 xcexcm.
Further, in a selective growth mask pattern for directly forming the active layer, open stripes for growing recombination layers to be inserted into a current blocking layer is formed besides the open stripe for growing the active layer, so that, by this mask pattern, the position and band gap of the recombination layers are controlled.
According to the present invention, it is ensured that, in an all-selective growth type BH laser which has excellent controllability, uniformity and reproducibility and in a method for the manufacture thereof, the position of band gap composition of a narrow band gap layer which is effective in improving the current confinement characteristic can be arbitrarily controlled for optimization thereof. As a result, the current confinement characteristic of the semiconductor laser is enhanced; and thus, semiconductor lasers with an excellent high-temperature high-output characteristic can be mass-produced at low manufacturing costs.